Toshihiko Kawamori

Epidemiological and laboratory studies suggest that nonsteroidal antiinflammatory drugs reduce the risk of colon cancer and that the inhibition of colon carcinogenesis is mediated through modulation of prostaglandin production by cyclooxygenase (COX) isozymes (COX-1 and -2). Overexpression of COX-2 has been observed in colon tumors; therefore, specific inhibitors of COX-2 activity could potentially serve as chemopreventive agents. Our recent study indicated that celecoxib (SC-58635), a specific COX-2 inhibitor, suppressed colonic aberrant crypt foci formation induced by azoxymethane in rats and led us to investigate more specifically the chemopreventive potential of this compound using colon tumors as end points. Five-week-old male F344 rats were fed the control diet (modified AIN-76A) or an experimental diet containing 1500 ppm celecoxib. Two weeks later, all animals except those in the saline-treated groups received s.c. injections of azoxymethane (15 mg/kg of body weight) once weekly for 2 weeks. All groups were kept on their regimen until the experiment was terminated, 50 weeks after carcinogen treatment. Colon tumors were evaluated histopathologically. Remarkably, dietary administration of celecoxib inhibited both incidence and multiplicity of colon tumors by about 93 and 97%, respectively. It also suppressed the overall colon tumor burden by more than 87%. The degree of tumor inhibition was more pronounced with celecoxib than it was with previously evaluated nonsteroidal anti-inflammatory drugs. The results of this study provide evidence, for the first time, that a specific COX-2 inhibitor, celecoxib, possesses strong chemopreventive activity against colon carcinogenesis.

Curcumin, derived from the rhizome of Curcuma longa L. and having both antioxidant and anti-inflammatory properties, inhibits chemically induced carcinogenesis in the skin, forestomach, and colon when it is administered during initiation and/or postinitiation stages. This study was designed to investigate the chemopreventive action of curcumin when it is administered (late in the premalignant stage) during the promotion/progression stage of colon carcinogenesis in male F344 rats. We also studied the modulating effect of this agent on apoptosis in the tumors. At 5 weeks of age, groups of male F344 rats were fed a control diet containing no curcumin and an experimental AIN-76A diet with 0.2% synthetically derived curcumin (purity, 99.9%). At 7 and 8 weeks of age, rats intended for carcinogen treatment were given s.c. injections of azoxymethane (AOM) at a dose rate of 15 mg/kg body weight per week. Animals destined for the promotion/progression study received the AIN-76A control diet for 14 weeks after the second AOM treatment and were then switched to diets containing 0.2 and 0.6% curcumin. Premalignant lesions in the colon would have developed by week 14 following AOM treatment. They continued to receive their respective diets until 52 weeks after carcinogen treatment and were then sacrificed. The results confirmed our earlier study in that administration of 0.2% curcumin during both the initiation and postinitiation periods significantly inhibited colon tumorigenesis. In addition, administration of 0.2% and of 0.6% of the synthetic curcumin in the diet during the promotion/progression stage significantly suppressed the incidence and multiplicity of noninvasive adenocarcinomas and also strongly inhibited the multiplicity of invasive adenocarcinomas of the colon. The inhibition of adenocarcinomas of the colon was, in fact, dose dependent. Administration of curcumin to the rats during the initiation and postinitiation stages and throughout the promotion/progression stage increased apoptosis in the colon tumors as compared to colon tumors in the groups receiving AOM and the control diet. Thus, chemopreventive activity of curcumin is observed when it is administered prior to, during, and after carcinogen treatment as well as when it is given only during the promotion/progression phase (starting late in premalignant stage) of colon carcinogenesis.

Accumulating evidence indicates that overproduction of prostanoids attributable to overexpression of cyclooxygenase-2 (COX-2) plays an important role in colon carcinogenesis. We have shown recently that the prostaglandin (PG) E receptor, EP(1), but not EP(3), is involved in mouse colon carcinogenesis. In line with our previous study, here we examined the role of prostanoid receptors in colon carcinogenesis using six additional lines of knockout mice deficient in prostanoid receptors EP(2), EP(4), DP, FP, IP, or TP. The animals were treated with the colon carcinogen, azoxymethane (AOM), and examined for the development of aberrant crypt foci (ACFs), putative preneoplastic lesions in the colon. Formation of ACFs was decreased only in the EP(4)-knockout mice, to 56% of the wild-type level. To confirm these results, we also examined the inhibitory effects of an EP(4)-selective antagonist, ONO-AE2-227, in the diet on the formation of AOM-induced colon ACFs in C57BL/6Cr mice and on the development of intestinal polyps in Min mice. ONO-AE2-227 at a dose of 400 ppm reduced the formation of ACFs to 67% of the control level, and intestinal polyp numbers in Min mice receiving 300 ppm were decreased to 69% of the control level. Plating efficiency assays showed that addition of 1.0 microM ONO-AE1-329, an EP(4)-selective agonist, resulted in a 1.8-fold increase in the colony number of the human colon cancer cell line, HCA-7, similar to the effect of PGE(2). Moreover, EP(4) mRNA expression was clearly observed in normal colon mucosa and colon tumors in mice. Our previous and present results indicate that PGE(2) contributes to colon carcinogenesis through its actions mediated through EP(1) and EP(4) receptors; therefore, antagonists for these two receptors may be good candidates as chemopreventive agents against colon cancer.

Although the cyclooxygenase pathway of the arachidonic acid cascade has been suggested to play an important role in colon carcinogenesis, the molecular species of prostanoids and receptors involved have not been fully elucidated yet. We examined the development of aberrant crypt foci (ACFs), putative preneoplastic lesions of the colon, in two lines of knockout mice, each deficient in prostaglandin E receptors, EP1 and EP3, by treatment with the colon carcinogen, azoxymethane. Formation of ACFs was decreased only in the EP1-knockout mice to approximately 60% of the level in wild-type mice. Administration of 250, 500, or 1000 ppm of a novel selective EP1 antagonist, ONO-8711, in the diet to azoxymethane-treated C57BL/6J mice also resulted in a dose-dependent reduction of ACF formation. Moreover, when Min mice, having a nonsense mutation in the adenomatous polyposis coli gene, were given 500 ppm ONO-8711 in the diet, the number of intestinal polyps was significantly reduced to 57% of that in the basal diet group. These results strongly suggest that prostaglandin E2 contributes to colon carcinogenesis to some extent through its action at the EP1 receptor. Thus, EP1 antagonists may be good candidates as chemopreventive agents for colon cancer.

The modifying effects of dietary administration of the plant phenolic antioxidants caffeic acid (CA), ellagic acid (EA), chlorogenic acid (CGA) and ferulic acid (FA) during the initiation phase on 4-nitroquinoline-1-oxide (4-NQO)-induced tongue carcinogenesis and on the number and area of silver-stained nucleolar organizer region proteins (AgNORs), a new cell proliferation marker, of the tongue squamous epithelium were investigated in male F344 rats. Rats were fed the diet containing 500 p.p.m. CA, 400 p.p.m. EA, 250 p.p.m. CGA or 500 p.p.m. FA for 7 weeks. One week after the commencement of the diets, 4-NQO (20 p.p.m.) was administered in the drinking water for 5 weeks. Feeding of four phenolic compounds significantly reduced the incidences of tongue neoplasms (squamous cell papilloma and carcinoma) and preneoplastic lesions (hyperplasia and dysplasia) by 32 weeks, and rats fed CA or EA had no tongue neoplasms. The number and area of AgNORs per nucleus were decreased significantly by dietary treatment with these four phenolics. Thus, CA, EA, CGA and FA inhibited the tongue carcinogenesis induced by 4-NQO when they were administered concurrently with the carcinogen. These results might suggest possible application of these natural substances for cancer chemoprevention in tongue in addition to other tissues (skin, lung, liver and esophagus).